One note that might be useful to speakers of Asian languages (not necessarily the asker, but a common related problem): we usually only use words like “delicious” or “tasty” to describe something AFTER we have judged it. “That food was delicious!” “Bob’s breakfasts are always tasty” “Wow! That looks so yummy!” We usually don’t use these words to describe food in the abstract: “I like to eat tasty food” and “Let’s go eat delicious food” both sound strange. For these sorts of sentences, use “good”. “I like to eat good food.” “Let’s go find something good to eat.”

Interesting question. I think it's fine for a woman ( of any age) to say 'yummy' with family and friends, and perhaps with other females e.g. women colleagues. In fact, you could say it in most contexts, providing you acknowledge the fact that you're using a word which we all know is childish. For example, someone could probably get away with telling a restaurant waiter that something was 'yummy', if it's accompanied by the right kind of smile and body language to show that you;re deliberately choosing informal language.

It is a word that a child with a limited vocabulary might use but with friends in the right setting it sounds appropriate by someone of any age.

I think there is a bit of (and dare I say) sexy appeal to a woman using the word the yummy. That is unless its around kids then the thing in question is just absolutely tasty. That is not to say though in the most concrete terms it is not okay for a man to think the use of the word yummy is a slight at something less than being in the purest sense something just playful. All Im really trying to say is that woman tend to feel more youthful when they use the word yummy. Men notice it but we dare not say anything. LOL. Don't hang me please. LOL. as an edit to the answer; one may say "that man is yummy" meaning they have an attraction to the man. But all other examples are going to be about food and so it is appropriate to use "yummy" in all examples for a woman. Another edit: It would be okay to say something like "the baby's feet are yummy" and then go about pretending to eat them. LOL again. Thank you for your question I am having way to much fun answering this.